Page 320 - Biology_F5

Gas exchange and respiration

The fate of pyruvic acid under aerobic presence of dehydrogenase enzyme and
respiration NAD.
In aerobic respiration, the pyruvate from The Kreb’s cycle
glycolysis is completely oxidised to carbon
dioxide and water using oxygen. In the first Task 6.6
stage, pyruvic acid is broken down to carbon Search from the internet sources the
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dioxide and hydrogen. This occurs in the simulations or videos on Krebs cycle
matrix of mitochondria and involves the including the step-by-step breakdown
Kreb’s cycle. In the second stage, hydrogen is of complex substances and then,
prepare short notes on what you have
oxidised by oxygen to form water in a series observed.
of reactions that constitute the respiratory
chain or electron transport system. This Kreb’s cycle is also known as citric acid
occurs on the cristae of the mitochondria. cycle. When oxygen is available the pyruvate
The transition between glycolysis and produced during glycolysis enters Kreb’s
Kreb’s cycle cycle, named after Sir Hans Adolf Krebs
(1900-1981), who worked out the details
There is a transitional stage between of the cycle in 1930. Kreb’s cycle takes
glycolysis and Kreb’s cycle or tricarboxylic place in the matrix of the mitochondrion.
acid (TCA) cycle. During this stage, each
pyruvic acid molecules enters the matrix of Steps involved in the Kreb’s cycle
the mitochondrion where it undergoes two The Kreb’s cycle consists of a series of
enzyme-catalysed reactions. It involves
types of reactions: the following steps:
(a) Decarboxylation, by losing a carbon atom
as carbon dioxide: The products of this The first step is the reaction of acetyl-

oxidative decarboxylation (acetyl) are CoA with oxaloacetate to form citrate,
in which the acetyl CoA (2C) is joined
carried by coenzyme A (CoA) resulting to oxaloacetate (4C) to form citrate (6C).
into formation of acetyl Coenzyme A This process requires the input of water,
(acetyl CoA). and it is catalysed by citrate synthetase
(b) Oxidation by dehydrogenation, in the enzyme.

Acetyl - CoA + Oxaloacetate + H O Citrate synthetase Citrate + Coa-SH

2
The second step is the formation of isocitrate. In the presence of aconitase enzyme, citrate is
The second step is the formation of isocitrate. In the presence of aconitase enzyme,
converted into isocitrate. This process is accomplished by dehydration and rehydration to yield
citrate is converted into isocitrate. This process is accomplished by dehydration and
an isomer called isocitrate.

rehydration to form an isomer called isocitrate.

Form Five Student’s Book
313
The third step of reaction is oxidation of Isocitrate. In this reaction the Isocitrate is oxidatively
decarboxylated to form α- ketoglutarate. The enzymes involved are isocitrate dehydrogenase
and oxalosuccinate decarboxylase together with NAD, and the products are NADH and CO .
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2

The fourth step is the oxidation of α- Ketoglutarate to form Succinyl – CoA. In the presence of
α- ketoglutarate dehydrogenase enzyme, α- ketoglutarate is oxidatively decarboxylated to form
+
succinyl CoA (4C). During this reaction NAD is reduced to NADH2. The products are
NADH and CO .
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